Preheat the oven to 225°F (107°C) and line a baking tray with parchment paper.
Whisk the egg whites, sugar and salt together in a metal bowl placed over a pot filled with an inch (2.5 cm) of gently simmering water. Warm the eggs to no more than 98°F (37°C), which helps the sugar to dissolve better as you whip the meringue.
Transfer the warmed egg whites and sugar to a stand mixer fitted with the whip attachment or use electric beaters to whip the whites to a stiff peak on high speed. Sift the cocoa powder overtop and fold it into the meringue, but only partly – you want to see streaks and ribbons of the cocoa powder and not have it fully mixed in.
Use two large kitchen spoons to make six big dollops of meringue on the baking tray. Bake the meringues for 2 hours, until they lift easily away from the parchment paper. Cool the meringues on the tray on a rack.
Notes
The meringues will keep in an airtight container at room temperature for 1 week.